The researchers followed up participants biennially in the prospective cohorts of the Nurses' Health Study (63,893 women; 1984-2012) and the Health Professionals Follow-up Study (41,094 men; 1986-2012). Eligible participants were 40 years or older, were free of POAG, and reported eye examinations.

Information on diet was updated with questionnaires. The researchers found that greater intake of dietary nitrate and green leafy vegetables was associated with a 20 per cent to 30 per cent lower POAG risk.

The association was particularly strong (40 per cent-50 per cent lower risk) for POAG with early paracentral visual field loss (a subtype of POAG linked to dysfunction in blood flow autoregulation).

"These results, if confirmed in observational and intervention studies, could have important public health implications," the researchers said. The findings were published in JAMA Ophthalmology.